Gstreamer eats 100% CPU in Ubuntu 16.04 on AWS

Annoying issue I happened to experience lately. Servers that we launch inside
AWS from Ubuntu 16.04 AMI had this gst-plugin-scanner process running and
eating 100% of one of CPU cores. Investigation revealed that this is a
gstreamer-related process which had no business running in a non-graphical
environment.

Apparently, installing some packages on top of the base AMI made Systemd switch
to graphical.target as the default target (runlevel in old terminology).
Graphical target starts things like Lightdm which consequently spawn different
GUI-related processes. And turns out some of those processes get their brains
fried.

The solution is to restore the default runlevel back to the multi-user one.

sudo systemctl set-default multi-user.target

Note that you must do this either when you build your custom AMI, or you can do
this on an already launched instance, but then reboot it.